Actin and Arp2/3 dynamics in chemotaxis. The cells shown respond to cAMP applied through a micropipet that is placed outside the frames shown. The direction of the diffusion gradients is indicated by arrows. (a) A cell labeled with full‐length LimE‐GFP recorded at a frame rate of 100 ms. (b and c) Cells labeled with GFP‐Arp3 (green) and mRFP‐LimEcoil (red). Yellow regions indicate merge of the two labels. The pattern of dense actin assemblies in (b) resembles the pattern in (a), showing that these structures are enriched in the Arp2/3 complex. At subsequent stages, Arp2/3‐rich actin structures are distributed along the region of the cell border that is exposed to higher cyclic‐AMP concentrations. (c) Arp2/3‐rich actin structures are preferentially accumulated at the base of filopods that point toward the source of the gradient. [In (b) and (c), network structures are not visible, because the resolution of dual‐emission fluorescence recordings is limited.) Time is indicated in seconds. (Scale bar: 5 µm.) (Reprinted with permission from Ref 69. Copyright 2005 National Academy of Sciences USA).

The 3D reconstructions of a representative Ax4 cell in the front (a), at the peak (b) and in the back (c) of a simulated temporal wave of cAMP generated in a perfusion chamber. Nonparticulate pseudopodial zones are demarcated in red. The cell is viewed at each time point at angles of 15° and 60° from the surface. Note that the Ax4 cell is elongated along the substratum in the front of the wave, rounds up and retracts the dominant pseudopod at the peak of the wave, and resumes pseudopod formation but in all directions and without cell elongation in the back of the wave. The behavior of this cell is representative of that of nine additional Ax4 cells reconstructed in 3D in a similar fashion. (Reprinted with permission from Ref 63. Copyright 2003 American Society for Microbiology).